Novel Approaches for Sustainable Management of Chromium Contaminated Wastewater

Abstract

The release of high volumes of untreated effluents containing different forms of chromium into waterbodies and further use of this wastewater for ferti-irrigation purposes pose a direct threat to health of human populations consuming produces from such agricultural fields. The higher concentration of chromium above permissible limits at these sites may pose harm to flora and fauna. The conventional processes used for treatment of chromium-containing effluents have low treatment efficiency, high operational costs, and produce toxic sludge requiring safe disposal. In contrast, the approaches exploiting use of living systems, such as microbes/microbial products and microbes, may provide sustainable treatment options. The emerging advanced/novel treatment technologies based on harnessing metabolic potential of microbiome of the polluted sites have potential to achieve the efficient removal of heavy metals from polluted sites. The success of protocols developed and tested at lab scale needs to be replicated at pilot/industrial to handle high volumes with varying levels of organic co-contaminants and harsh physiological conditions. The presented chapter provides an overview of impact of high chromium levels on ecosystem and various treatment processes with advanced aspect of management of heavy metals to prevent harmful effects on the environment.

1. Introduction

Due to rapid globalization and industrialization, there is an extensive use of chromium (Cr) in various industries involved in the production of steel and chromium alloys, leather tanning, textile industries, and wood preservation for meeting the need of increasing population. Various types of Cr-containing waste are being generated viz. tannery effluent, generation of chromite mining waste leading to soil and groundwater contamination. In light the overall cost including environmental cost of chromium mining it is imperative to develop technologies for recovery/recycling of Cr from such wastes. The release of partially/untreated wastewater containing heavy metals, such as chromium (Cr), from various industrial units causes the pollution in the water and sediments of aquifers/waterbodies [1]. Industrially, Cr is mostly used in steel manufacturing, electroplating and chrome plating, leather tanning, textile dyeing, and wood manufacturing. The higher/extensive use of these heavy metals has considerably exacerbated the problems related with water pollution, which has negative effects on economy, and public health-related issues worldwide. Additionally, the use of water from these contaminated waterbodies for agriculture purpose led to accumulation of heavy metals and other pollutants in agricultural crops affecting food safety, human health, and deterioration of ecological niches [2]. Chromium (Cr) is the tenth most common element in the Earth’s mantle, with an atomic number of 24 and a relative atomic mass of 51.9. Most of the Cr in soil/water environment is found in the two most common valence states, that is. Cr (VI) and Cr (III). Among them, Cr (VI) is having more aqueous solubility than as compared to Cr (III) state and resultantly more mutagenic and carcinogenic than later [3]. The pollution related to the presence of Cr (VI) in underground aquifers can pose risk to flora and fauna of the receiving waterbodies. Additionally, the consumption of Cr (VI) polluted water can cause adverse health effects among the human population such as skin infections, respiratory diseases, kidney disorder, liver-related disorder, and cardiovascular diseases [4]. Further, the use of wastewater for ferti-irrigation caused accumulation of these pollutants in agricultural soil and direct exposure of human population consuming produces from such sites. Cr (VI) can be toxic to both prokaryotic and eukaryotic cells as it can be easily transported into the cells by the process of facilitated diffusion, using nonspecific anion channels. Inside the cell, Cr (VI) can be reduced to pentavalent, tetravalent, and trivalent forms by various enzymatic systems. Cr (VI) at higher concentrations can form oxyanions/hydroxyl radicals causing strand break in DNA structure [5].

The commonly used methods for lowering/removal of Cr (VI) from effluents/wastewater are based on physical/chemical methods such as adsorption (activated carbon and renewable agricultural waste), ion exchange, membrane filtration, electrochemical methods, hydroxide precipitation, sulfide precipitation, and chelating precipitation, etc. The use of physical/chemical processes has numerous disadvantages such as recurring cost of consumables, high operation cost, and formation of toxic sludge requiring proper disposal in sanitary landfills. In light of this, there is an urgent need to develop sustainable treatment strategies, which might be less expensive than existing treatment options and do not produce toxic sludge/byproducts, which may need cost intensive handling processes. Additionally, a treatment option with possibility of recovery of metals for their reuse in process will make the treatment options more attractive for end users and will be more environmentally friendly. The treatment methods based on use of biological tools, such as plants and microorganisms, are attractive options being used. However, the accumulation of metals in plant and microbial biomass poses a problem in both extraction of metals of their safe disposal. The more appropriate options being conversion of the toxic form to less toxic form by reduction or precipitating out the metals from effluents or in situ metal precipitation (ISMP) on sludge or soil matrix, resulting in lowering of their toxic potential. Further, the recent reports on use of microbial products viz metallothioneins, siderophore, exo-polymers, etc. to bind or sequester out metals have provided other options, which may contribute to economic growth in the metal use and recycling [6, 7]. Biological system requires metabolically versatile inocula, which may be able to maintain their potential under harsh conditions such as temperature, pH, and nutrients for successful treatment [8]. Recently, with the advent of novel/advanced treatment technologies, including nanotechnology, combined treatment technologies, and synthetic biology tools, such as micro-biome engineering and systems biology, have provided a wide field to develop more sustainable options than other treatments for the restoration of ecosystems affected by presence of high level of heavy metals. The application of bioengineering and molecular biology tools can facilitate the transfer of treatment processes developed at laboratory or pilot scale to field scale level. The establishment of multiple levels of validation of bioremediation will open a new window for easier audit of bioremediation processes [1].

This chapter outlines application of Cr (VI) in various industries, its environmental, and health impact as well as management of chromium contaminated wastewater using conventional, biological, and advanced methods. This chapter also serve as valuable source for scientist and researchers working in remediation of Cr (VI) present in industrial wastewater to protect our environment and ecosystem.

2. Chromium-based industries and pollution

Chromium finds its applications in many industries, especially in steel manufacturing, textile and dyes, leather, and chemical manufacture, such as chromated copper arsenate (CCA), used in wood preservation. Many of these industrial facilities use low-grade chromite ores for a variety of purposes such as producing pigments, polishing metal, preventing corrosion, synthesizing organic materials, tanning leather, and preserving wood as shown in Figure 1 [9]. The leather industry alone accounts for 40% of the worldwide usage. Chromium is used as a tanning agent for making leather durable and less susceptible to decomposition. Chromium is also used in the production of steel and chromium alloys to improve their hardening and corrosion resistance [10]. Chromium is used in wood preservation in the form of chromated copper arsenate (CCA) to impregnate wood products so that it can be protected from insects and termites [11]. The tetrahalides, CrF₄, CrCl₄, and CrBr₄ of chromium are used as mordant and catalyst in dyeing processes of chrome dyes in textile industries. The manufacture/synthesis processes from abovementioned products may produce a large amount of solid wastes and effluents rich in different forms of chromium with other components with pollution potential. Large number of heavy metal contaminants, including Cr (VI),are produced from industries such as tannery processing, chromate preparation, metal preparation, production of the colors chrome and ferrochrome, and temper steel welding [12].

Figure 1.

Consumption of chromium in different industries.

The most important cause of chromium contamination in the environment is the release of untreated wastewater from various industries involved in either the production or utilization of chromium. Additionally, the disposal of toxic sludge from various industries and treatment plants can cause soil pollution, ultimately percolating to groundwater sources. Many of industries release varying amount of heavy metals found in their wastewater, which are released into the environment either directly or indirectly [13]. Due to the wide variations in the nature of these wastewaters depending on the type of industry, treatment of these wastewaters is a major topic of concern for achieving removal of heavy metals. In addition to this, leaching and weathering of chromite ores reserves can lead to major contamination of soil and water, causing environmental concern [14].

3. Impact of high level of chromium on environment and human health

In recent times, due to unrestricted discharge by point and nonpoint sources of varied industries, heavy metal, such as Cr (VI), caused irreparable damage to environment and animal/human health. The persistent release of Cr (VI) contaminated wastewater has led to its buildup in water and soil environment, causing adverse effects on flora and fauna. Some of these effects/impacts of Cr (VI) are discussed in the section below.

3.1 Cr (VI) impact on environment

Due to extensive use of Cr (VI) in many applications causes unprecedented damage to the environment. Recent studies have shown the accumulation of Cr (VI) beyond permissible limits in groundwater sources, wastewater effluents, and soil environments. In plants, Cr (VI) can be used for development, productivity, and metabolism [15]. However, plants exposed to higher concentrations of Cr (VI) had been reported for adverse effects on their physiology, morphology, and structural makeup. The nutrient, as well as water uptake mechanisms, may be affected, resulting in slowing down the growth and photosynthetic activity [16]. In literature, Wang et al., [17] studied that the exposure of maize plant to 300 mg/l of Cr (VI) as K₂Cr₂O₇ can cause the alterations in leave morphology of maize plant. Pandey et al., [18] reported inhibition of electron transport in photosystem-I and photosystem-II in presence of high concentration of Cr (VI) in which plants system. Also, the higher concentrations of Cr (VI) can cause oxidative damage to DNA and membrane lipids, as well as chlorosis and necrosis in plants [19].

The accumulation of Cr (VI) in soil may affect the overall productivity of agricultural crops as it had been reported that 94% reduction in stem growth was evident in plants of 32 species exposed to 1000 mg/kg plant weight as compared to control plants without exposure to any Cr (VI) [20]. The Cr (VI) bioaccumulation had been reported in fish, aquatic plants, algae, and invertebrates leading to hazardous and toxic effects on overall growth and development [21]. It was reported that accumulation of Cr (VI) in aquatic system cause increase in Cr (VI) level in higher trophic levels of food chains due to the phenomenon of bioaccumulation. This can lead to more harmful consequences in terms of toxicity in higher trophic level of food chain. More importantly, the potential risk of Cr (VI) has to studied in more depth in order to evaluate its concern regarding wildlife and environmental damages [22].

3.2 Cr (VI) impact on human health

Cr (VI) is reported to be highly soluble in water as compared to Cr (III) form, which can lead to its absolute presence in water sources. Lower concentrations of Cr (VI) are necessary for number of bodily functions such as lowering blood cholesterol level by decreasing low-density lipoproteins (LDLs), maintaining blood sugar level and as chromium picolinate used in supplements for gaining muscle mass [23, 24]. However, at higher concentrations, it can lead to cytotoxic and genotoxic effects on animal cells. Accidental breathing of Cr can cause irritation in the lungs and nose. Further, it can lead to lung cancer, kidney dysfunction, asthma, ulcers, and diarrhea [25]. Acute exposure can lead to choking, wheezing, and shortness of breath, whereas chronic exposure can lead to development of ulcers and perforations in respiratory, gastrointestinal, bronchitis, septum parts, etc. In literature, Cr (VI) can cause damage to blood cells, causing kidney and liver failure and also can lead to toxicity in bloodstream due to its oxidative nature [26]. In light of the serious effects of environment and overall health of flora and fauna, including human beings due to high levels of different forms of chromium, there is an urgent need to develop environmentally friendly and sustainable treatment systems for wastes/effluents having their high levels. Hence, due to severe environmental damage and its harmful health effects, we should design protocols for the treatment of effluents containing Cr (VI) as a pollutant.

4. Conventional treatment technologies

Cr (VI) can be removed from wastewater using variety of methods, including physical/chemical and biological methods. Various physical/chemical methods, which can be used for the treatment of Cr (VI) containing wastewater are described below.

4.1 Physical methods

Various physical methods, such as adsorption, ion exchange, membrane filtration, and electrochemical removal, can be used for the removal of Cr (VI) from wastewater.

4.1.1 Adsorption

Adsorption method is a surface deposition based on phase transfer process to remove chromium from contaminated water. It is based on the principle of purification or sorption of metal onto the pores of the sorbent, which reduces the concentration of toxic metal ions from contaminated wastewater [27]. Adsorption process is simple to carry out and cost-effective for the treatment of wastewaters. For adsorption, various types of adsorbent materials can be used, such as activated carbon and agricultural waste, as mentioned below [28].

4.1.1.1 Activated carbon

The most widely used adsorbent materials for the removal of Cr (VI) from wastewater is activated carbon. Different materials, such as coconut shell, wood, and sawdust, can be used for making activated carbon [27]. Activated carbon can be classified into two forms, that is. powdered activated carbon (PAC) and granular activated carbon (GAC) [29]. Commercially available activated carbon includes: Filtrasorb-100, F-400 (commercially activated carbon) and GA-3 [30, 31]. The advantage of using activated carbon includes low price, high surface area (500 to 1500 m²/g), and easy operation as compared to conventional methods [27]. The high adsorption capacity of activated carbon depends on pH and temperature during the adsorption process. An increase in temperature was reported to decrease the adsorption capacity for the removal of chromium, whereas a low pH was reported to increase the adsorption capacity of activated carbon [32]. Moreover, activated carbon needs to be chemically activated to enhance its adsorption capacity and should be regenerated after each treatment cycle [27].

4.1.1.2 Use of agricultural waste

Agricultural waste or its byproducts can be a good option to remove Cr (VI) from wastewaters, for example, pine needles, cactus [33], timber, grain crops, peanut shells, hazelnut shell, bark, and tea leaves [34]. There are various mechanisms used in bio-sorption, such as chemisorption or complexation, with pores present on its surface due to adsorption and ion exchange [35]. The advantage of this method is that the adsorbent material is derived from low-cost agricultural waste, which can be used for the recovery of metals such, as Cr (VI) [27]. However, the main disadvantage of this technique is that plant material cannot be directly used without chemical pretreatments (using acid or base), for example, treatment of orange peels with calcium chloride (CaCl₂) and sodium hydroxide (NaOH), can result in increased adsorption capacity because of the transformation of methyl esters as inhibiting groups to carboxylate ligands. As a result, the metal binding capability was greatly enhanced. The metal uptake can be increased by increasing number of binding sites, better ion-exchange ability, and formation of new functional groups. The application of these chemical pretreatments might increase the cost of treatment hindering its use for long-term treatment [34, 35, 36].

4.1.2 Ion exchange-based removal

Ion exchange methods are based on removal of chromium using a support having resins for ion exchange. The support materials used in ion exchange columns can include Dowex 2X4 [37], Ambersep 132 [38], solvent-impregnated resin with quaternary ammonium salt (aliquat 336), polyacrylate anion exchanger having strong base functional groups Amberlite IRA 458, and Amberlite IRA 958 [39]. The Cr (VI) containing wastewater was made to pass through resin bed under pressure, where chromium is removed by ion-exchange mechanism. When resin capacity is exhausted due to binding of chromium, the column is backwashed to remove trapped solids and then it can be regenerated using HCl [40]. Ion exchange method requires low maintenance. However, it is expensive, susceptible to fouling, and readily clogged by organic materials and other compounds found in wastewater. In general, pretreatment is required before using ion exchange resin on wastewater that contains significant quantities of suspended particles and salts, thereby limiting its application for the treatment of industrial wastewater having Cr (VI) as a pollutant [39]. The use of acid for the regeneration of ion exchange columns makes this process more expensive. Additionally, the release of this acid after the regeneration process from ion exchangers can cause secondary pollution [9, 40].

4.1.3 Membrane filtration

The fundamental component of membrane processes is the use of a semipermeable membrane to divide a solution into two distinct streams. The use of different membranes for the treatment of wastewater containing Cr (VI) determines the separation selectivity. There are many types of membranes, which can be used for the removal of Cr (VI) from wastewater such as polymeric, inorganic, and liquid membranes. The inorganic membranes are very costly, but they are chemically and thermally stable [27]. The polymer-based membrane filters made up of polyethersulphone with carboxymethyl sulphonate (CMC) as complexing agent was found to achieve 99.5% efficiency of 10 mg/l of Cr (VI). However, these polymer membranes generally face biofouling problem [41]. Liquid membranes have high diffusion rates, but they instable due membrane expansion and liquid loss during the removal of Cr (VI) [27]. The main disadvantage of using membrane filtration is its high cost and recovery process for the removal of Cr (VI) is not efficient.

4.1.4 Electrochemical methods

It is also referred to as electro-dissolution or electrocoagulation method. The combination of electrochemical and chemical reactions makes this method of choice to be used for the treatment Cr (VI) present in solid, liquid, and gaseous matters [42]. Electrocoagulation process uses two different electrodes namely: anode (sacrificial electrode) that is usually aluminum or iron and a cathode (liberates hydrogen gas) as shown in Figure 2 [43]. Electrochemical removal process has two stages: [1] Reduction of Cr (VI) to Cr (III) and [2] Precipitation as hydrated chromium complex. The electro-dissolution method involves dissolution of iron (anode) under strong acidic condition followed by reduction of Cr (VI) as shown in reactions of Figure 2 [44]. It is widely used in electroplating industry to treat industrial wastewater, but its efficiency is affected by the presence of other ion, especially salt ions and change in pH.

Figure 2.

Electrocoagulation of chromium.

Reaction mechanisms of chromium reduction in electrochemical cell are as follows:

$$\text{FeO}+{\text{2H}}_2\text{O}\to {\text{Fe}}^{2+}+{\text{2H}}_2+{\text{O}}_2+{\text{2e}}^{\_}$$E1

$${\text{6Fe}}^{2+}+{\text{Cr}}_2{\text{O}}_7{}^{2-}+14{\text{H}}^{+}\to {\text{6Fe}}^{3+}+{\text{2Cr}}^{3+}+{\text{7H}}_2\text{O}$$E2

4.2 Chemical precipitation methods

Chemical precipitation is used as a common method for the removal of chromium from industrial wastewater. Chemical precipitation is a relatively simple process in which different chemicals react with heavy metals to form insoluble precipitates. These precipitates can be separated from the water by sedimentation or filtration, followed by its decantation for appropriate discharge [45]. The main disadvantages of chemical precipitation include high operational and maintenance cost, energy input, manual oversight, and generation of large sludge [34]. Chemical treatment of wastewater containing heavy metals includes different methods as described below.

4.2.1 Hydroxide precipitation

Hydroxide precipitation is the most commonly used method among chemical precipitation. It is based on the principle that metal hydroxide complex, which can be formed by the addition of Ca (OH)₂ or NaOH in wastewater containing heavy metals. These metal hydroxide complexes can be removed by the process of sedimentation or flocculation [46]. In literature, it was reported that the addition of 4% Ca (OH)₂ achieved complete removal of 30 mg/l of Cr (VI) at a pH of 8.3 [47]. The advantage of using hydroxide precipitation is that it is simple and low-cost method used for the removal of heavy metals from industrial effluents. The disadvantages of hydroxide precipitation include generation of large amount of sludge due to amphoteric nature of hydroxides, and it can create problem in maintaining pH of the wastewater, which can inhibit metal hydroxide precipitation.

4.2.2 Sulfide precipitation

Sulfide precipitation is used for the removal of heavy metals from wastewater by the addition of iron sulfide or pyrite [48]. Sulfide precipitation is non-amphoteric in nature, so it does not require maintenance of pH for the removal of heavy metal. The main disadvantage of this process includes evolution of H₂S fumes and formation of colloidal precipitates that cause separation problems [34].

4.2.3 Chemical precipitation with other methods

Chemical precipitation (i.e., hydroxide and sulfide precipitation) can be used with other methods, such as nano-filtration or with ion exchange method in order to overcome some of the limitations of hydroxide or sulfide precipitation method. Sulfide precipitation can be used with nano-filtration for the removal of heavy metals. Ion exchange method can be used in combination with chemical precipitation for the removal of heavy metals. But the combination of these methods can be expensive as acid used in regeneration of ion exchange columns can cause secondary pollution [41].

4.2.4 Chelating precipitation

This method uses chelating precipitation for the removal of heavy metals from wastewater. N-bis-(dithiocarboxy) piperazine (BDP) and 1,3,5- hexahydrotriazinedithiocarbamate (HTDC) were used for the removal of complex heavy metal from wastewater containing Ni²⁺, Cr⁶⁺ and Cu²⁺ [41]. Both BDP and HTDC can effectively lower the concentration of different heavy metals in wastewater to less than 0.5 mg/l. In literature, it was reported that the use of potassium ethyl xanthate (KEX) for the complete removal of 1000 mg/l of Cu²⁺ from wastewater [49]. All the above mentioned methods are conventionally used for the removal of Cr (VI) as large volumes can be treated in short time; however, they have several disadvantages. The use of physicochemical methods requires specific conditions for proper treatment and can be affected by interferences of other pollutants, which actually reduce their efficiency for proper removal of Cr (VI). Moreover, generation of higher quantities of sludge, high operational costs, and secondary pollution caused due to the release of chemicals/acids used in the treatment process make them environmentally unsustainable [50]. The various advantages and disadvantages of various physical/chemical methods are given in Table 1.

Technique	Method	Advantages	Disadvantages
Physical methods	Adsorption	Porous sorbent hence high capacity. Suited for highly contaminated sites. Effective and fast kinetics.	Laborious and costly. Performance depends upon type of carbon used. Complexing agent requires to improve its performance.
	Ion exchange method	Energy efficient, effective removal.	Costly as required backwashing and regeneration of column. Causes secondary pollution.
	Use of agricultural waste	Low cost. Convenient raw material. Little environmental pollution and easy recovery of metals.	Agricultural waste requires chemical pretreatments may increase the cost of treatment and may result in biomass loss. Causes secondary pollution.
	Membrane filtration	Less space required and specific in action. Low energy demand.	Membrane fouling expensive process. Require proper maintenance.
	Electrochemical methods	Low cost and high selectivity. No additional chemical required.	Sludge production. Costly and require energy consumption and maintenance.
Chemical precipitation	Hydroxide precipitation	Rapid, easy, and cost-effective method.	It generates large amount of sludge. Maintenance of pH is required for effective removal. Complexing agent can inhibit metal removal. Causes secondary pollution.
	Sulfide removal	Rapid, easy, and more effective than hydroxide precipitation as pH maintenance is not needed.	Formation of hydrogen sulfide. Formation of colloidal precipitates that cause separation problems.
	Combination of methods	Rapid, easy, and more effective	Secondary pollution, Expensive process
	Chelating precipitation	Easy and effective at lower concentration of heavy metals.	Generate large amount of sludge, Secondary pollution.

Table 1.

Advantages and disadvantages of various physical and chemical methods used for the treatment of Cr (VI) from wastewater [34].

5. Bioremediation: an eco-friendly approach

The biological treatment for achieving removal/reduction of chromium from wastewater provides an environmentally sustainable alternative to conventional treatment system. The use of microorganisms, such as bacteria, fungi, and yeast to reduce the toxic hexavalent chromium at polluted site to relatively less-toxic trivalent chromium, is an attractive option. Different microbial species, in mixed or pure cultures, are being reported for wide array of enzymes, which can achieve reduction of Cr (VI) [8]. These microbes belong to both aerobic and anaerobic domains and can work effectively at interface of such environment, thus providing multitude of options for natural bioremediation processes. Different groups of microorganisms can be used for the removal of Cr (VI) from wastewater, and these include the following.

5.1 Bacteria

Bacteria are diverse group of microorganisms, which can grow/divide rapidly and possess array of different enzymes. As per various literature reports, bacterial cells and their enzymes can reduce Cr (VI) to Cr (III), which is a significantly less hazardous metal [51]. Cr (VI) is mutagenic and toxic to bacteria, but it was observed that gram-positive bacteria are more resistant to Cr (VI) than gram-negative bacteria because of their thick cell wall. In literature, it was reported that 10–12 mg/l of Cr (VI), inhibited most soil bacteria in liquid media, such as Pseudomonas aeruginosa CCTCC AB93066, whereas Cr (III) at this concentration was found to be nontoxic [52, 53].

The reduction of Cr (VI) can be carried by either pure culture or by mixed bacterial culture. There are various reports for the treatment of Cr (VI) by bacterial isolates but only few of them can perform under both aerobic and anaerobic conditions [54]. Most of the studies for the reduction of Cr (VI) were carried out using pure microbial culture. In literature, various bacterial isolates were reported for the reduction of Cr (VI), for example, Cellulosimicrobium sp. isolated from common effluent treatment plant (CETP) of a tannery plant, which was able achieve 97% reduction of 100 mg/l Cr (VI) after 72 h incubation [9]. Similarly, Banerjee et al. [55] isolated Bacillus strain TCL from Tasra coalmine lake (TCL), Jharia (Jharkhand), which was able to achieve complete reduction of 200 mg/l Cr (VI) in Luria-Bertani (LB) broth after 16 h incubation. Zeng et al. [56] isolated Oceanobacillus sp. W4 from chromium-contaminated soil, which was able to achieve 74.2% reduction of 200 mg/l Cr (VI) after 72 h incubation. Similarly, Acinetobacter junii strain b2w was able to achieve 98.24% reduction of 10 mg/l Cr (VI) to Cr (III) form. This was due to the involvement of various mechanisms bioaccumulation and efflux pumps used for the removal of Cr (VI) by Acinetobacter junii strain b2w [57].

However, the efficiency of pure cultures under field-scale applications can be affected by their survival under extreme environmental conditions and establishing themselves in competition with other predominating bacterial species already adapted to such conditions [58]. Hence, mixed bacterial consortium might have a better chance to survive during field-scale studies as compared to pure culture [59]. The metabolic versatility of different groups of microorganisms can complement each other to achieve better cleanup efficiency as compared to that achieved by individual pure cultures [60]. Bacteria can use different mechanisms by which it can achieve removal of heavy metals such as bioaccumulation, bio-sorption, bio-leaching, bio-transformation based detoxification, or in situ metal precipitation (ISMP) based on the presence of enzymes, metallothioneins and/or exopolysaccharides. Singh et al. [61] reported an SRB (sulfate reducing bacteria) consortium, which was able to achieve 96% reduction of 50 mg/l Cr (VI) in Postgate growth medium supplemented with 2.58% sodium lactate as carbon source using a batch type small scale bioreactor. Ma et al. [58] isolated mixed bacterial consortium comprising Aeromonas, Pseudogracilibacillus, and Acinetobacter, which was able to achieve complete reduction of 10 mg/l Cr (VI) in acetate minimal medium after an incubation of 72 h.

5.2 Fungi

Fungi, mostly yeasts isolated from different chromium-contaminated sites, had been reported for their resistance to high levels of chromium (VI). This metal resistance was attributed to the presence of cell wall and production of different enzymes, which carry out extracellular and intracellular precipitation, redox reactions, and active intake into the cells [62]. In literature, Xu et al. [63] isolated Paecilomyces lilacinus XLA, which was able to achieve 96.86% reduction of 100 mg/l Cr (VI) in minimum mineral (MM) medium after an incubation of 14 days. Long et al. [64] isolated Penicillium oxalicum SL2, which was able to achieve 89.6% reduction of 217 mg/l Cr (VI) from electroplating wastewater after an incubation of 96 h. Igeihon and Babalola, [65] isolated Penicillium sp., which was able to achieve 50% reduction of 16.1 mg/l Cr (VI) in Norkrans medium after an incubation of 20 days. Kumar and Dwivedi [66] isolated Trichoderma lexii, which was able to achieve 99.4% reduction of 50 mg/l Cr (VI) in potato dextrose broth medium after an incubation of 120 h. Hence, the expulsion of Cr (VI) was reported to be prominent in fungi.

5.3 Algae

The photosynthetic organisms, such as cyanobacteria or algae, have a ubiquitous distribution, and there are reports regarding their tolerance or resistance of algae to Cr (VI) [67]. The resistance to Cr (VI) in algae was attributed to presence of polysaccharides, proteins, or lipids in their cell wall, which can act as binding agent of heavy metals as it contains carboxyl, sulfate, amino, and hydroxyl groups. Some studies suggested sequestration of Cr (VI) by its complexation with organic compounds in algal exudates/biomass [68]. Megharaj et al. [69] reported suppression of algal growth when it was exposed to 40 mg/kg of Cr (VI). Gupta and Rastogi [68] isolated Oedogonium hatei from freshwater, which was reported for bio-sorption of 30 mg/g of Cr (VI) from 100 mg/l Cr (VI) aqueous solution. Das et al. [70] reported a salt-tolerant micro-alga Chlorella vulgaris NIOCCV for its ability to achieve complete reduction of 3.22 mg/l of Cr (VI) from tannery wastewater after 12 days incubation. Costa et al. [71] isolated Chlorella vulgaris, which was able to achieve complete reduction of 10 mg/l Cr (VI) in the presence of photocatalyst experiment carried with titanium dioxide (TiO₂) after an incubation of 16 h.

5.4 Cyanobacteria

Cyanobacteria also known as blue-green algae have been reported for affinity to heavy metals due to their higher extracellular mucilage content [72]. Garnham and Green [73] reported the accumulation of 10 nmoles of Cr (VI)/gm dry weight by unicellular non-nitrogen-fixing cyanobacterium Synechococcus sp. PCC 6301. However, filamentous nitrogen-fixing cyanobacterium Anabaena variabilis was able to achieve 20% reduction of 10 nmoles Cr (VI) to Cr (III). This reduction of Cr (VI) by A. variabilis was due to the presence of heterocysts, and so it was able to tolerate 100 mg/l of Cr (VI), whereas the growth of Synechococcus sp. PCC 6301 was completely inhibited at this concentration of Cr (VI) [73]. Parveen et al. [74] reported Synechocystis sp. strain PUPCCC 62, which was able to reduce 250 nmoles of Cr (VI) per mg of protein. Sen et al. [72] isolated a cyanobacterial consortium having Limnococcus limneticus and Leptolyngbya subtilis, which was able to achieve 50% reduction of 15 mg/l Cr (VI) in BG-11 medium after 12 days. These studies indicate that cyanobacteria can also be used for the removal of Cr (VI) (Table 2).

Group	Culture name	Site of isolation	Reduction efficiency	Reference
Bacteria	Pseudochrobactrum saccharolyticum LY10	Chromium-contaminated soil	95% removal of 130 mg/l Cr (VI)	[75]
	Brevibacterium sp. K1	Soil sample	43.6% reduction of 500 mg/l Cr (VI)	[76]
	Stenotrophomonas sp. D6	Soil sample	87.6% reduction of 500 mg/l Cr (VI)	[76]
	Sporosarcina saromensis M52	Sediment sample	Complete reduction of 100 mg/l Cr (VI)	[77]
	Bacillus cereus (TIB3) strain	Mine tailings	82% reduction of 100 mg/l Cr (VI)	[78]
	Bacillus strain TCL	Coalmine lake	Complete reduction of 200 mg/l Cr (VI)	[55]
	Anoxybacillus flavithermus ABF1	Geothermal region	93.71% reduction of 20 mg/l Cr (VI)	[79]
	Bacillus paramycoides Cr6	Soil under the chrome slag of a chromate plant	100% reduction of 200 mg/l of Cr (VI)	[80]
Fungi	Fusarium	Tannery effluent	Complete removal of 10 mg/l Cr (VI)	[81]
	Penicillium commune	Soil and sludge sample	73% reduction of 9.86 mg/l Cr (VI)	[82]
	Trichoderma asperellum strain PTN7	Soil sample	Able to reduce 2.7 mg/l of Cr (VI)	[83]
	Trichoderma asperellum strain PTN10	Soil sample	Able to reduce 8.35 mg/l of Cr (VI)	[83]
	Penicillium oxalicum SL2	Air contaminated with industrial vapors	Complete reduction of 200 mg/l Cr (VI)	[64]
	Trichoderma lexii	Electroplating water	99.4% reduction of 50 mg/l Cr (VI)	[66]
Algae	Spirogyra	National chemical laboratory	98% bio-sorption of 5 mg/l Cr (VI)	[84]
	Chlorella vulgaris	Tannery wastewater	Reduce 3.22 mg/l of Cr (VI)	[70]
Cyanobacteria	Nostoccalcicola HH-12	Metal contaminated soil	Complete bio-sorption of 20 mg/l Cr (VI)	[85]
	Chroococcus sp. HH-11	Metal contaminated soil	Complete bio-sorption of 20 mg/l Cr (VI)	[85]
	Limnococcus, Limneticus and Leptolyngbya subtilis consortium	Wetland	50% removal of 15 mg/l Cr (VI)	[72]

Table 2.

Bio-reduction of chromium by different groups of organisms.

6. Advanced removal methods

Recently, advanced approaches are developed with the goal of enhancing effectiveness and sustainability for the elimination of Cr (VI) from wastewater. These methods are essential for addressing the serious environmental and health issues associated with heavy metal pollution. A few of them are mentioned below.

6.1 Using nanotechnology approach

The field of nanotechnology can be used for the removal of heavy metals from industrial wastewaters. It is a very efficient method for the removal of heavy metals from wastewaters due to their stability and small size, large surface area, and easily accessible pore space [86]. In addition to this, due to their small size, they possess efficient Brownian motion properties, making them suspended in wastewater for removal of heavy metals, such as Cr (VI). The use of nanoparticles in conjunction with other adsorbent materials, such as ion exchange resin and zerovalent ions, can increase the bioremediation potential for the removal of heavy metals, such as Cr (VI), present in wastewaters [87, 88]. Currently, various types of nanomaterials can be used for the uptake of heavy metals, such as Cr (VI), present in wastewater such as metal-organic framework (MOFs), metal oxides, carbon materials, and chitosan. Nithiya et al. [89] reported 55.71 mg/g removal of Cr (VI) based on adsorption phenomenon using chitosan/silicagel-based nano-composites. Recently, carbon nano-tubes (CNTs) can be used for the removal of heavy metals from wastewater due to their unique hollow structure, outside surface area, and high activity due to fast transport of water. Further, the surface modification of CNTs with groups, such as hydroxyl and carboxylic acid, can increase solubility of CNTs in wastewater due to high hydrogen bonding capacity, ion exchange, and increasing functionalization using nanoparticles increasing affinity toward heavy metals [90, 91]. In literature, Lyu et al. [88] reported adsorption of 130 mg/g of Cr (VI) using a biochar-supported nanoscale iron sulfide (FeS) composite (CMC-FeS@biochar) having three components viz. biochar, carboxymethyl cellulose (CMC), and FeS acting as adsorbent.

6.2 Combined treatment technologies

Combined treatment methods include the use of two techniques to achieve the efficient removal of heavy metals, such as Cr (VI). The removal of chromium from wastewater various approaches such as chemical precipitation, adsorption, membrane filtration, biological treatment, electrocoagulation, and advanced oxidation processes [86, 92]. These methods can be used alone or in combination to effectively remove chromium depending on the specific characteristics of the wastewater and regulatory requirements. Certain systems are designed not solely for the purpose of chromium removal, but also with the intention of recycle it for reuse. Pilot-scale testing is typically done to determine the most suitable treatment approach for a given wastewater stream [13]. As electrochemical and biological treatment can be used in culmination for the removal of heavy metals. In literature, Suganthi et al. [93] reported the removal of color and chemical oxygen demand (COD) present in tannery wastewater by using hybrid membrane bioreactor having activated sludge and electrocoagulation. Hou et al. [94] reported 51.3% removal of overall Cr and 48.1% of hexavalent chromium using electrochemically operated bio-sorption framework followed by adsorption using Sargassum sp. Similarly, Moradi and Moussavi et al. [95] reported 100 and 98.27% removal of COD and absolute Cr and sulfide from tannery wastewater using the combination of UVC/VUV photobioreactor and electrocoagulation technique. Hence, combination of two or more techniques can be used for efficient removal of heavy metals present in industrial wastewaters.

6.3 Use of synthetic biology tools for improving microbial potential

Synthetic biology tools use combination of molecular biology tools combining systems and cell biology to design/synthesize genes, enzymes, or transcription factors to redesign metabolic pathways for achieving targeted studies. Some of the methods, that is, microbiome engineering and synthetic biology are used nowadays to design synthetic microbes, specifically for achieving bioremediation are described in detail below.

6.3.1 Microbiome engineering

The interaction between microbial population and heavy metals is the basis of bioremediation of metal contaminated environments. The presence of diverse autochthonous microbial population can help in mitigation of heavy metals [96]. Microbiome includes different groups of micro-organisms capable to achieving bioremediation of heavy metals. The knowledge regarding microbiome of polluted environments and synthetic biology can pave ways for the removal of heavy metals from complex wastewater environments [97]. Nowadays, molecular biological tools (MBTs) can be used to identify contaminant degrading capabilities of microbiome present at polluted sites and further designing bioremediation protocols. The microbiome present in soil or wastewater environment can replenish the microenvironments using different methods such as immobilization, reduction of heavy metals to less toxic forms, and binding to cellular entities [98]. However, these activities depend upon efficiency of cultures present, nutrient sources, and physical and environmental factors. The activity of these autochthonous population can be increased by alteration in any of these factors. Also, manipulations of genes can increase siderophore, metallothionine production, and exo-polymer production increasing mitigation of heavy metals. In addition to this, efficient indigenous microbial culture can be used to achieve the desired bioremediation of wastewaters and soil environment [99].

6.3.2 Synthetic biology

The field of synthetic biology allows scientists to design and construct genetically engineered microorganisms capable of achieving reduction Cr (VI) to less toxic Cr (III). The use of interdisciplinary fields, such as biochemistry, genetic, and systems biology can plays a pivotal role in enhancing the efficiency of bioremediation processes using micro-organisms [100]. The field of biochemistry apprises various mechanisms by which microorganisms bind, sequester, or transform heavy metals and also focuses on the characteristics of various biomolecules, such as peptides or synthesis of metal-chelating compounds to achieve metal-binding proteins [92]. It can unravel various enzymes responsible for achieving reduction of heavy metals, such as Cr (VI). The knowledge regarding metabolic pathways and enzymes can be used to construct engineered microbes with enhanced abilities to reduce heavy metals to less toxic forms [101]. To achieve this, various genetic techniques such as gene knockout, overexpression or introduction of genes coding for metal-binding proteins, transporters proteins, efflux pumps, or enzymes can be employed to modify microorganisms for increased biotransformation capabilities [92, 100, 101, 102]. In literature, Xue et al. [103] engineered Pseudomonas putida KT2440 with self-controlled genetic circuit sensing variation in the concentration of mercury (Hg²⁺). Nowadays, various gene editing methods can be used such as ZFN (zinc finger nuclease), TALENs (transcription activator-like effector nucleases), and CRISPRs (clustered regularly interspaced short palindromic repeats) used as transcription activators. Also, CRISPR-Cas9 approach, which can be precisely used to engineer microorganisms for specific heavy metal bioremediation applications [104].

In recent times, systems biology can help develop biosensors providing real-time monitoring to detect the presence of heavy metals in wastewater or polluted environments [105]. In literature, Adekunle et al. [106] described the use of floating microbial fuel cells (MFCs) as biosensors for the detecting the presence of copper (Cu) as low as 35–40 μg/L and other heavy metals present in aquatic environments. Systems biology can provide holistic understanding of complex biological processes, including microbial communities that can play an important role in bioremediation of heavy metals. It includes the identification of microbial species, their functions, and understanding complex interplay/interactions among themselves and environmental factors to optimize bioremediation processes [107]. Also, mathematical modeling and simulation can predict the behavior of microbial communities in response to changing environmental conditions to design and optimization of bioremediation strategies. By integrating experimental data with mathematical models, researchers can develop efficient strategies to remove chromium from wastewater. Systems biology can also be used to monitor the impact of chromium bioremediation on the surrounding environment [108]. The ongoing development of innovative technologies and interdisciplinary collaboration will continue to drive progress in the field of bioremediation.

7. Conclusion

Chromium is widely used in several industrial processes as an important component, which leads to its pervasive presence in wastewaters of industrial origin. Further, the release of these wastewaters into rivers and drains leads to heavy metal presence in water resources causing harm to inhabiting flora and fauna. This leads to increase in the presence of heavy metals pollution in the environment (water/soil). The environment, human health, and the ecosphere are all at risk when excessive levels of heavy metals are released into the air, water, and soil. This chapter discusses pollution related to Cr (VI), and its harmful effects on environment and human health. We have also highlighted the treatment/removal of Cr (VI) using various conventional techniques, having many drawbacks; in contrast, biological treatment offers better alternative for the treatment of Cr (VI) from industrial wastewaters. In bioremediation aspect, different microorganisms, such as bacteria, fungi, algae, and cyanobacteria, can use a variety of mechanisms, such as metal sequestration or reduction for the removal of Cr (VI). In addition to this, advanced removal methods, including nanotechnology, combined treatment technologies, and synthetic biology tools, such as microbiome engineering and systems biology, can be used as one of the efficient and sustainable method for the removal of heavy metals from wastewater. Hence, the removal of Cr (VI) from wastewater is an important need of time in order to prevent harmful effects on environment and human health.